doc: rename writing_tests.md to writing-tests.md

The writing_tests.md file did not utilize kebab-case as the other files
in the directory did.

PR-URL: https://github.com/nodejs/node/pull/9867
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

CONTRIBUTING.md

@@ -147,7 +147,7 @@ $ git rebase upstream/master
 
 Bug fixes and features **should come with tests**. Add your tests in the
 `test/parallel/` directory. For guidance on how to write a test for the Node.js
-project, see this [guide](./doc/guides/writing_tests.md). Looking at other tests
+project, see this [guide](./doc/guides/writing-tests.md). Looking at other tests
 to see how they should be structured can also help.
 
 To run the tests on Unix / OS X: